You just need to have a decent knowledge of how the steem blockchain works, knowledge of how to make an iOS app and a solid plan of what you want to develop.

To learn more about the steem blockchain, you can check out the steem developer portal: https://developers.steem.io/. They give a lot of the details, and if you don't understand something, I recommend checking out the steemdevs discord here: https://discord.gg/ytwt3AD. Theres even a dedicated swift page if you will be using swift to make your iOS app.

You should also know how to make an iOS app, that means having an apple developers account and knowing what tools to use. You can go with react native(which I haven't used so I won't talk about that) or go native. Steemit Inc has made their own library to interface with the steem blockchain with iOS and it can be found here: https://github.com/steemit/swift-steem. There's also swiftyconnect which is a steemconnect library for iOS development developed by @moonrise. Its still a work in progress and can be found here: https://github.com/caspernikus/SwiftyConnect. You should also know Swift or Objective-C to make the apps natively if you can.

And finally you need to know what you are developing. You should have a solid idea of what your final app will be and what the reason for it. This can be the hardest part and you should spend time drawing the flow of the app out on paper. Good luck.